The most common materials currently utilized in manufacturing soft contact lens are polymers and copolymers of 2-hydroxyethyl methacrylate (HEMA). These hydrophilic polymers move well on the eye and provide sufficient oxygen permeability for daily wear. Some HEMA soft contact lenses have been approved for extended periods of wear up to 7 days. However, such extended wear may result in comeal swelling and development of surface blood vessels in the sclera.
Research for improved oxygen permeable polymers has led to the development of polymers containing silicone groups. A variety of siloxane-containing polymers exhibit high oxygen permeability. Because of their oxygen permeability siloxane-containing polymers show great promise as the next generation of contact lens polymer. Unfortunately, siloxane-containing polymers possess physical characteristics that have thus far hindered their ascension to dominance in the field of contact lenses.
In layman's terms, siloxane-containing polymers are sticky. Contact lenses made of these polymers are hydrophobic and tend to adhere to various surfaces, severely complicating the manufacturing process. For example, a siloxane-containing lens will adhere to surfaces during the transfer of the lens from point to point during the manufacturing process. One particular point in the manufacturing process that often causes problems is removing the lens from the mold, a step that is also known as “deblocking” the lens.
Those familiar with the art know that a contact lens mold typically consists of a base curve (convex) mold half and a front curve (concave) mold half formed from a polymer. In the siloxane-containing lens manufacturing context, polyolefin (e.g., polypropylene) molds are most commonly used. The front curve and base curve mold halves are fitted together to form a small crescent shaped mold cavity between the base curve mold half and the front curve mold half. Introducing a fluid monomer to the front curve mold and then sandwiching the monomer with the base curve mold forms a fluid monomer in the shape of a lens. The choice of monomer and the shape of the crescent shaped cavity determine the optical properties of the lens. The monomer is then polymerized through heat treatment, light treatment or other polymerizing process, thus forming a soft contact lens.
After the lens is formed the mold halves are separated. Contact lenses, especially siloxane-containing lenses, regularly stick to one of the mold halves. In the siloxane-containing lens context, the lenses tend to attach to the front curve mold half. Those skilled in the art typically refer to a front curve or back curve mold half as a “mold.” For the rest of this discussion the terms mold half and mold will be used interchangeably unless the context requires otherwise. Those skilled in the art will readily recognize such context.
The reason for the particular attachment to the front curve mold is not completely understood. The adherence of the lens is probably related to a combination of the lens mold interface phenomena and physical properties of the lens including the mold surface morphology, internal stress build up within the lens (or distortion) and the stress distribution and the wettability of the lens material.
Under dry conditions it is difficult to separate a siloxane-containing lens from a mold surface due to adhesion between the lens and mold surface. Lenses can be forced to separate from the mold surface by applying a force, such as with a pair of tweezers. Nevertheless, the application of such force to peel a lens off of a mold surface often results in damage to the lens. For example, the lens may become scratched, distorted or torn, each of which renders it useless.
The adhesion between the lens and mold surface can be weakened when the molecules of the lens polymer become mobile. For example, the molecules may become mobile by adding heat or chemicals such as a solvent. Accordingly, an alternative method for removing a lens from a mold surface involves the use of a solvent such as isopropyl alcohol (“IPA”). In this method IPA is applied directly to the lens as it adheres to the mold surface. The solvent swells the lens and helps reduce the forces holding the lens to the mold surface. The lens may then be removed from the mold surface.
Although this method of deblocking reduces the likelihood of damage to the lens, the collection and disposal of used solvent carries both an economic and environmental price. For example, used IPA may be classified as hazardous waste in some states. Accordingly, a need exists for an improved method for removing a contact lens from a mold.